Exposure to carcinogens through diet, air, and other means is generally regarded to influence human cancer risk, but the impact of specific environmental carcinogens on human breast cancer incidence is still unknown. We examined whether distinct chemical carcinogens induce a unique transcriptional profile in mammary gland cancer that is characteristic of the etiologic agent. Rat mammary gland cancers (N=34) were generated by various carcinogens including food-derived heterocyclic amines 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, 7,12-dimethylbenz[a]anthracene, N-nitrosomethylurea, and 4-aminobiphenyl. The histopathology of the carcinomas was graded through a modified Scarff-Bloom Richardson scheme, and the gene expression profiles in the carcinomas were evaluated on a 10K cDNA microarray. Unsupervised hierarchical clustering analysis revealed two major clusters of carcinomas irrespective of the carcinogenic agent that distinguished two groups with different histopathologic parameters (degree of differentiation, nuclear grade, mitotic activity, epithelial cell growth pattern and necrosis). Using class comparison analysis and hierarchical clustering of all carcinomas irrespective of histopathology, gene expression profiles were further shown to be statistically differentially expressed according to the carcinogenic agent. These findings indicated that the transcriptional program in carcinomas is unique for the etiologic agent and can be observed among a diverse set of carcinogens despite variations in carcinoma histopathology. The ability to use microarray analysis to discern an etiology-specific profile among a pathologically heterogenous group of breast carcinomas may ultimately be valuable for determining the role of environmental chemical carcinogens in human breast cancer risk. Signal transducer and activator of transcription (Stat) 5a is a transcription factor mediating the action of specific cytokines, growth factors and hormones on gene expression. In the mammary gland, Stat5a is well recognized for its function in prolactin signaling, lobuloalveolar development, and milk protein expression during pregnancy and lactation. Latent cytoplasmic Stat5a is activated by tyrosine phosphorylation and following dimerization undergoes nuclear import. Stat5a expression was examined immunohistochemically in carcinomas induced by the chemical carcinogens 7, 12-dimethylbenz[a]anthracene and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. A high percentage of carcinomas showed nuclear labeling of Stat5a [44 of 68 (65%)] with Stat5a nuclear labeling index ranging from 18-77%. In contrast, control normal mammary gland tissue displayed cytosolic expression. Carcinomas with different Stat5a staining patterns (cytoplasmic or nuclear) showed a statistical difference for the proliferating cell nuclear antigen (PCNA) labeling, tumor differentiation, nuclear grade, mitotic activity, and tumor size. High Stat5a nuclear expression was closely correlated with the higher-grade, more poorly differentiated carcinomas. Stat5a nuclear expression was also detected in the intraductal proliferation (10 of 21 lesions) and in ductal carcinoma in situ (13 of 15 lesions). Immunohistochemical analysis was further carried out in human breast cancers. Stat5a nuclear expression was detected in ductal and lobular carcinomas and DCIS at a frequency of 48% (15/31), 33% (2/6), and 40% (2/5), respectively. Nuclear expression of Stat5a in human breast cancers also correlated with PCNA nuclear labeling index. The findings implicate activated Stat5a in mammary gland cancer development in the rat and human.The expression of estrogen receptor alpha (ERalpha), estrogen receptor beta (ER beta) and progesterone receptor (PR) was examined in mammary gland carcinomas induced by PhIP in female Sprague-Dawley rats.